Radio transmitters in most communication systems are required to be “linear”, meaning that their output signal must accurately reproduce a frequency spectrum of a corresponding input signal. A non-linearity in the input/output relationship causes the output signal to spread onto adjacent radio frequencies and interfere with other radio channels. Pre-distortion is a technique of cancelling an initial non-linearity by altering the input signal to produce an overall linear input/output relationship. In a pre-distorted transmitter, a pre-distortion function applied to the input signal is adjusted based on a feedback signal. Since the distortion primarily occurs in the power amplifier, the signal acquired after the power amplifier is fed back and compared with the initial signal as part of the pre-distortion process.
Currently available pre-distorted transmitters are generally designed to operate for a single frequency band or a small range of transmitted frequencies (e.g. LTE band 1 or 2110-2170 MHz). Both a forward path and a feedback path in these pre-distorted transmitters are effectively tuned to the same frequency band, and cannot be deployed to support other frequency bands. The pre-distorted transmitters' operation in a single (narrow) frequency band of interest occurs as a result of a design which includes, for example, various tuned circuits (e.g., narrow band filters), a limited range of operation of components (e.g., voltage controlled oscillators that only tune only over a few hundred MHz), and amplitude and phase variation over frequency which makes linearization (e.g., pre-distortion) difficult over a broad range of frequencies.
The number and range of available frequency bands around the world has recently increased significantly. The currently available pre-distorted transmitters require that they be redesigned, modified and re-banded to operate in a new frequency range. This situation leads to many radio communication systems which are expensive to design and to support on an on-going basis.
Accordingly, it would desirable to have a frequency agile pre-distorted transmitter capable of being effectively tuned for plural frequency bands, thereby avoiding the afore-described problems and drawbacks.